1. Field of the Invention
The present invention relates to the field of data storage and retrieval devices. In particular, the present invention relates to the mitigation of head instability in such devices.
2. Description of the Prior Art
As the data storage industry has pushed towards higher and higher data storage densities, the magneto-resistive (MR) sensor sensitivity has progressively increased with a concomitant decrease in the sensor size to support higher tracks per inch (TPI) requirements. These changes have made the data reader sensitive to noise phenomena which are manifestations of kinks and loops (also called bifurcation loops) that are present in the media field range of the MR transfer curve. The loops and kinks in the transfer curve, which give rise to the observed head instability, are eventually classified as baseline popping (BLP), permanent magnet reversal instability (PMRI), spiking noise, writer instability, or random telegraph noise (RTN). The observed effect in all head instability cases is extra pulses, missing pulses and thermal asperity (TA) like events whose time constant is related to the alternating current (AC) coupling capacitor in the preamplifier. Any external factor that changes the kinks and loops in the transfer curve or the location of the same on the MR transfer curve could result in the disappearance or reappearance of spikes and BLP events. Whenever these bifurcation loops occur in the media field sweep range, instability can occur due to thermal excitation that causes jumps to either side of the bifurcation loop in the transfer curve. When the jumps occur to one side of the bifurcation loop (which is enclosed by the media field range while the other side is not), it results in positive or negative spikes. When the bifurcation loop is completely enclosed by the media field range, it causes a random baseline shift that is the result of jumps from one side of the loop to the other due to thermal excitation.
Bias current level, temperature, mechanical stress resulting from the head fabrication processes, and time all have an effect on the MR transfer curve and the location of loops and kinks within the transfer curve. As has been mentioned previously, problems only occur when the loops and kinks lie within the media field sweep range. Because the loops shift around and may move outside the media field range at certain bias current temperature levels, it is possible that at certain temperature and bias current ranges, the head is impervious to the aforementioned instability phenomena. The time constant and the nature of these jumps determine whether they affect drive performance. The servo pattern in the drive usually bears the brunt of the effects of the instability.
Although the root causes of head instability are well known, it is difficult to screen for the phenomena effectively at any given stage of the drive fabrication process without compromising good heads. In the prior art, a combination of techniques can be used to cope with head instability.
The various conventional techniques that can be used to deal with head instability cover a broad spectrum of solutions from screening at the head level to adjusting the channel response. Screening measures attempt to eliminate head instability whereas other techniques attempt to deal with head instabilities through changes to the drive manufacturing process or various drive adjustments. A few of these conventional techniques include the following:                1. Head Screening—HSA or HGA level screen that uses the magnetic transfer curve to look for loop anomalies.        2. Temperature Sensitive Bias and Write Current Optimization—Optimizes the bias and write current over a range of temperatures to find write current and bias settings which are less prone to instability.        3. Adjusting the preamp AC coupling capacitance—A method of adjusting the time constant of the head instability to help the drive cope with the instabilities.        4. Adjust the High pass pole of the Partial Response system—A method to track out low frequency head instability events by adjusting the High pass pole of the band-pass Partial Response transfer function.        
Although these prior art techniques have some effect in dealing with head instability, a less expensive and more reliable solution is needed.